The use of noble metals with great activity being costly, attempts have been made to have them as highly dispersed on a carrier as possible, to make nearly all the atoms of the metal accessible to the reactants (atomic dispersion). In practice, when using conventional methods of manufacture, for example impregnation with a salt of the selected metal, it is difficult to obtain very high degrees of dispersion. Relatively big metal crystallites are obtained, with a diameter which is often larger than 30 .ANG. (30.times.10.sup.-10 m).
As relates to bifunctional catalysts having an acid function and a metal function, such as, for example, reforming or hydrocracking catalysts, ionic exchange methods have been found, which provide very high metal dispersions.
Unhappily the carriers to be used are generally of large surface: alumina of the gamma cubic type with a specific surface of about 200 m.sup.2 /g, silica-aluminas or zeolites with even higher surfaces. Moreover it is necessary in these methods to use competing ions such as the chloride ion from hydrochloric acid which is used, for example, for manufacturing reforming catalysts. These competing ions avoid a chromatographic effect when impregnating the catalyst particle and thus lead to a homogeneous distribution of the metal within the particle. The residual presence of some of these competitor ions on the catalyst surface, and the usually acidic properties of the large surface carriers make these catalysts unsuitable to hydrogenation and dehydrogenation reactions since they have a low selectivity and also activate parasitic reactions of polymerization, isomerization and hydrocracking.
More recently, techniques have been found to graft organometallic complexes on inorganic or polymeric substrates. This type of technique has been called supported homogeneous catalysis. These methods lead easily to atomic dispersions of the metal phase. However they cannot be used easily and they often necessitate the use of reducers, such as organoaluminum compounds, which leave residues on the terminated catalyst which can activate parasitic reactions. The use of polymeric carriers results in swelling when hydrocarbons are present; and these catalytic systems are not regenerable by the conventional combustion methods.
The technique of the invention copes with these difficulties. It results in very high metal dispersions, such that at least 50% of the deposited metal atoms are accessible to the reactants, which is estimated to correspond to crystallites of a size smaller than 20 .ANG. (20.times.10.sup.-10 m).